60 Hz electric fields and incandescent light as aversive stimuli controlling the behavior of rats responding under concurrent schedules of reinforcement

Several reports have shown that animals will sometimes engage in behaviors that reduce their exposure to a 60 Hz electric field (E-field). The field, therefore, can function as an aversive stimulus. In other studies, the E-field at equivalent strengths failed to function as an aversive stimulus. The present experiment, using rats, demonstrates how factors other than field strength can influence whether a subject engages in behavior that reduces field exposure. The general design consisted of giving the rat a choice between two alternatives, one of which sometimes included an added stimulus. Each subject was trained to press each of two levers to obtain food. Pressing one lever was reinforced intermittently under a variable interval 2 min schedule (VI 2); pressing the other lever was reinforced by a second VI 2 schedule operating independently of the first. Under this concurrent schedule the rat spent 50% of the daily 50 min session responding to each of the levers, indicating that they were equally “valued.” Next, while the schedules remained in effect, the first response to one of the levers turned on a 100 kV/m E-field which remained on until the rat pressed the other lever. The time spent responding under the schedule associated with the field was reduced by about 5–10%. When the procedure was changed so that no lever presses produced food, i.e., extinction, but the added stimulus contingency remained, the rats spent even less time in the presence of the field. Similar outcomes were observed during both the concurrent food or extinction schedules when incandescent light was used. Thus, both an E-field and incandescent light functioned as aversive stimuli, but the magnitude of the aversiveness was small. Aversiveness depended not only on stimulus intensity, but also on behavioral factors. Bioelectromagnetics 19:210–221, 1998. © 1998 Wiley-Liss, Inc.     

An application of the active time model to multiple concurrent variable-interval schedules

The current experiment investigates whether an active time model can account for anomalous results that have emerged from multiple schedule, concurrent variable-interval (VI) VI experiments. The model assumes that (1) during concurrent VI VI training pigeons learn a function that relates time since the most immediate response, i.e., active time, to changeover probabilities and (2) that molar preference is the result of an interaction between inter-response time frequencies and the learned active time changeover functions. Pigeons were trained under a concurrent VI 30-s VI 30-s schedule and a concurrent VI 60-s VI 60-s schedule. Probes were conducted in which VI 30-s and VI 60-s stimuli were paired. During these probes, birds allocated choices equally to the stimuli. The active time model accurately fit individual subject data. In contrast data were not fit by a variant of scalar expectancy theory proposed by Gibbon [Gibbon, J., 1995. Dynamics of time matching: arousal makes better seem worse. Psychon. Bull. Rev. 2, 208-215].     

A further application of the active time model to multiple concurrent variable-interval schedules

In this experiment we show that the active time model (ATM) accurately predicts probe data from multiple concurrent VI VI schedules. Subjects were trained under a concurrent VI 30-s VI 60-s and a concurrent VI 60-s VI 120-s schedule. Two types of unreinforced probes were then conducted. The first paired the two VI 60-s stimuli. These stimuli, while equivalent in their associated absolute rates of reinforcement, differed in their relative rates of reinforcement. The second probe paired the VI 30-s stimulus with the relatively rich VI 60-s stimulus. In contrast with the first probe, these stimuli differed in their absolute rates of reinforcement, while being similar in their relative rates. During the first set of probes, birds preferred the VI 60-s stimulus trained with the VI 120-s schedule. During the second set of probes, birds were indifferent to the two stimuli. These results are less extreme than others reported in the literature. Nonetheless, we found that ATM accurately fit individual subject data in both sets of probes. In contrast a variant of scalar expectancy theory did not fit the data at either the individual or group level.     

Within-session changes in responding during concurrent variable interval variable ratio schedules

Rats (Experiment 1) and pigeons (Experiment 2) responded on several concurrent variable interval (VI) variable ratio (VR) schedules. The rate of, but not the time spent, responding in each component usually changed within-sessions. The bias and sensitivity parameters of the generalized matching law (GML) did not change systematically within-sessions. The fit of the GML to the data did not change within-sessions for pigeons, but it was better in the middle than at the beginning or end of the session for some for rats. Both over- and under-matching occurred. These results imply that within-session changes in responding do not usually cause problems for assessing the validity of the GML when subjects respond on concurrent VI VR schedules. The results also suggest that under- and over-matching are not produced by different factors, but rather lie on a continuum.     

Rapid acquisition of preference in concurrent schedules: effects of reinforcement amount

Four pigeons were trained on concurrent variable-interval 30-s schedules. Relative reinforcer amounts arranged across the two alternatives were varied across sessions according to a pseudorandom binary sequence [cf., Hunter, I., Davison, M., 1985. Determination of a behavioral transfer function: white-noise analysis of session-to-session response-ratio dynamics on concurrent VI schedules. J. Exp. Anal. Behav. 43, 43-59]; the ratios (left/right) were either 1/7 or 7/1. Reinforcer amount was manipulated by varying the number of 1.2s hopper presentations. Sessions ended after 30 reinforcers (15 for each alternative). After approximately 30 sessions, response ratios for all pigeons began to track the changes in amount ratio (i.e., subjects' responding showed a moderate increase in sensitivity of responding to reinforcer amount). Characteristics of responding were similar to procedures in which reinforcer rate and immediacy have been manipulated, although sensitivity estimates for amount were lower than those previously obtained with rate and immediacy. This procedure may serve as a useful method for studying the effects of certain environmental manipulations (e.g., drug administration) on sensitivity to reinforcer amount.     

Overshadowing of responding on ratio and interval schedules by an independent predictor of reinforcement

This experiment replicated previous demonstrations that interposing a brief stimulus between reinforced responses and the presentation of the reinforcer reduces responding maintained by intermittent reinforcement schedules. Furthermore, we could find no significant difference between the relative size of the reduction during training on ratio and interval schedules when the predictive significance of the response and stimulus was controlled by a yoking procedure.     

Variability of force and interresponse time under random interval reinforcement schedules

Response variability was examined under five random interval schedules with minimum mean interreinforcement intervals of 15, 30, 60, 120, and 240 sec. Distributions of response force-time integrals showed no systematic variability changes with increases in reinforcement intermittency. Corresponding distributions of interresponse times revealed increases in variability accompanying increases in reinforcement intermittency. The relationship of variability to these two measures, and the potential effect of behavior outside the experimentally measured response class, on recorded variability within the class, are considered.     

Contingency discriminability and the generalized matching law describe choice on concurrent ratio schedules of wheel-running reinforcement

Belke (2010) showed that on concurrent ratio schedules, the difference in ratio requirements required to produce near exclusive preference for the lower ratio alternative was substantively greater when the reinforcer was wheel running than when it was sucrose. The current study replicated this finding and showed that this choice behavior can be described by the matching law and the contingency discriminability model. Eight female Long Evans rats were exposed to concurrent VR schedules of wheel-running reinforcement (30s) and the schedule value of the initially preferred alternative was systematically increased. Two rats rapidly developed exclusive preference for the lower ratio alternative, but the majority did not - even when ratios differed by 20:1. Analysis showed that estimates of slopes from the matching law and the proportion of reinforcers misattributed from the contingency discriminability model were related to the ratios at which near exclusive preference developed. The fit of these models would be consistent with misattribution of reinforcers or poor discrimination between alternatives due to the long duration of wheel running.     

Reinforcement omission in concurrent fixed-interval and random-interval schedules

The present experiment examined overall and local effects of omission of reinforcers in a choice situation. Pigeons' key-pecking responses were reinforced under concurrent fixed-interval and random-interval schedules of food presentation. After some weeks of baseline sessions in which the probability of reinforcement was 1.00, approximately 25% of food presentations from the fixed-interval schedule were omitted and replaced by timeout periods. In such omission sessions, the overall relative rates of responding to the fixed-interval schedule became lower than those in the baseline sessions. On the other hand, when relative rates of responding to the fixed-interval schedule in the omission sessions were calculated separately for fixed-interval cycles preceded by timeout periods and those preceded by food presentations, the relative rates in the former type of fixed-interval cycles were higher than those in the latter type for three out of four pigeons. These results mean that relative rates of responding cannot always be regarded as reflecting a relative value of an alternative, and that the overall effect of the omission of fixed-interval reinforcers is not reducible to the local effect of omission.     

Rapid acquisition of preference in concurrent schedules: Effects of d-amphetamine on sensitivity to reinforcement amount

In the present study, effects of d-amphetamine on sensitivity to reinforcement amount under concurrent schedules were examined using a rapid-acquisition procedure. Four pigeons key pecked under single concurrent variable-interval 30-s schedules of grain presentation. Two different reinforcer-amount ratios (7:1 and 1:7) changed across sessions according to a 31-step pseudo-random binary sequence (PRBS). After at least four times through the PRBS, response ratios generally tracked the session-to-session changes in amount ratios; estimates of sensitivity ranged from 0.26 to 0.31 across the four pigeons. Effects of a range of doses of d-amphetamine (0.3-5.6 mg/kg) then were determined. For 3 of 4 pigeons, at least one dose, which did not dramatically alter overall response output or bias, decreased sensitivity to reinforcement amount. These results suggest that reducing sensitivity of responding to reinforcement amount may be one behavioral mechanism of stimulants, which may have implications for interpreting drug effects on self-control.     

Self-control and impulsiveness with asynchronous presentation of reinforcement schedules

IN DISCRETE TRIALS, PIGEONS WERE PRESENTED WITH TWO ALTERNATIVES: to wait for a larger reinforcer, or to respond and obtain a smaller reinforcer immediately. The choice of the former was defined as self-control, and the choice of the latter as impulsiveness. The stimulus that set the opportunity for an impulsive choice was presented after a set interval from the onset of the stimulus that signaled the waiting period. That interval increased or decreased from session to session so that the opportunity for an impulsive choice became available either more removed from or closer in time to the presentation of the larger reinforcer. In three separate conditions, the larger reinforcer was delivered according to either a fixed interval (FI) schedule, a fixed time (FT) schedule, or a differential reinforcement of other behavior (DRO) schedule. The results showed that impulsive choices increased as the opportunity for such a choice was more distant in time from presentation of the larger reinforcer. Although the schedule of the larger reinforcer affected the rate of response in the waiting period, the responses themselves had no effect on choice unless the responses postponed presentation of the larger reinforcer.     

Non-contingent positive and negative reinforcement schedules of superstitious behaviors

The role of schedules of reinforcement on the development of superstitious conditioning was investigated in a college age population. Participants were randomly assigned to one of eight operant schedules and instructed to remove (escape), prevent and/or remove (avoidance and escape) or produce (positive) the appearance of a computer generated stimulus using a response pad. Results from the experiment indicate that concomitant (escape and avoidance) schedules of reinforcement are most effective in facilitating acquisition of superstitious behavior as measured by self-reports of participants.     

"The stone which the builders rejected...": Delay of reinforcement and response rate on fixed-interval and related schedules

The article deals with response rates (mainly running and peak or terminal rates) on simple and on some mixed-FI schedules and explores the idea that these rates are determined by the average delay of reinforcement for responses occurring during the response periods that the schedules generate. The effects of reinforcement delay are assumed to be mediated by a hyperbolic delay of reinforcement gradient. The account predicts that (a) running rates on simple FI schedules should increase with increasing rate of reinforcement, in a manner close to that required by Herrnstein's equation, (b) improving temporal control during acquisition should be associated with increasing running rates, (c) two-valued mixed-FI schedules with equiprobable components should produce complex results, with peak rates sometimes being higher on the longer component schedule, and (d) that effects of reinforcement probability on mixed-FI should affect the response rate at the time of the shorter component only. All these predictions were confirmed by data, although effects in some experiments remain outside the scope of the model. In general, delay of reinforcement as a determinant of response rate on FI and related schedules (rather than temporal control on such schedules) seems a useful starting point for a more thorough analysis of some neglected questions about performance on FI and related schedules.     

Effects of ethanol on punished and nonpunished responding under conditions of equated reinforcement rates and similar response rates

Lever pressing by pairs of rats was maintained under random-ratio (first subject) and yoked-interval (second subject) schedules of food presentation. The inter-reinforcement intervals generated under the ratio schedule comprised the interval values for the second (yoked) subject. This arrangement yielded nearly equivalent rates of food presentation for each subject pair. For the first rat of each pair a random-ratio schedule of shock presentation was added to the ratio schedule of food presentation. This manipulation resulted in similar rates of punished (first rat) and nonpunished (second rat) responding within subject pairs. Ethanol administration (0.25–1.5 g/kg) generally resulted in dose-related decreases in both punished and nonpunished responding. In general, punishment-specific effects were not obtained. These results suggest that ethanol may not be as effective as chlordiazepoxide or pentobarbital in increasing punished responding even when the effects of baseline response and reinforcement rates are controlled.     

Interactions between T cells responding to concurrent mycobacterial and influenza infections

CD4(+) T cells are central in mediating granuloma formation and limiting growth and dissemination of mycobacterial infections. To determine whether T cells responding to influenza infection can interact with T cells responding to Mycobacterium bovis bacille Calmette-Guérin (BCG) infection and disrupt granuloma formation, we infected mice containing two monoclonal T cell populations specific for the model Ags pigeon cytochrome c (PCC) and hen egg lysozyme (HEL). These mice were chronically infected with PCC epitope-tagged BCG (PCC-BCG) and acutely infected with HEL epitope-tagged influenza virus (HEL-flu). In these mice, PCC-BCG infection is much more abundant in the liver than the lung, whereas HEL-flu infection is localized to the lung. We observe that both T cells have access to both inflammatory sites, but that PCC-specific T cells dominate the PCC-BCG inflammatory site in the liver, whereas HEL-specific T cells dominate the HEL-flu inflammatory site in the lung. Influenza infection, in the absence of an influenza-specific T cell response, is able to increase the activation state and IFN-gamma secretion of PCC-BCG-specific T cells in the granuloma. Activation of HEL-specific T cells allows them to secrete IFN-gamma and contribute to protection in the granuloma. Ultimately, infection with influenza has little effect on bacterial load, and bacteria do not disseminate. In summary, these data illustrate complex interactions between T cell responses to infectious agents that can affect effector responses to pathogens.     

Resistance to extinction, generalization decrement, and conditioned reinforcement

This study investigated generalization decrement during an extinction resistance-to-change test for pigeon key pecking using a two-component multiple schedule with equal variable-interval 3-min schedules and different reinforcer amounts (one component presented 2-s access to reinforcement and the other 8s). After establishing baseline responding, subjects were assigned to one of the two extinction conditions: hopper stimuli (hopper and hopper light were activated but no food was available) or Control (inactive hopper and hopper light). Responding in the 8-s component was more resistant to extinction than responding in the 2-s component, the hopper stimuli group was more resistant to extinction compared to the Control group, and an interaction between amount of reinforcement, extinction condition, and session block was present. This finding supports generalization decrement as a factor that influences resistance to extinction. Hopper-time data (the amount of time subjects spent with their heads in the hopper) were compared to resistance-to-change data in an investigation of the role of conditioned reinforcement on resistance to change.